FIELD OF THE INVENTION
The invention relates to a feeder of a paper sheet processing machine, such as a printing unit of a printing machine.
Deliveries of paper sheet processing machines have been heretofore known in which a paper sheet is taken from the stack and lifted up with the aid of lifting suckers. The sheet is then transferred to further transport means disposed downstream, as seen in a sheet transport direction, which deliver the paper sheet into paper processing units, for instance a printing unit of a printing machine. It is thereby important that the lifting suckers remain in the raised position until the rear edge (trailing edge) of the sheet has been removed from the stroke region of the lifting sucker. The lifting suckers are subsequently quickly lowered for picking up the next paper sheet.
Mechanical locking devices are known with which the lifting sucker is retained above the paper sheet and moved downwards after the retainer is released. Such mechanically intricate arresting systems are too sluggish, particularly for quick-moving machines. The establishment, and primarily the release, of the lock requires a very long time, so that the next paper sheet can only be lifted up belatedly, which substantially impairs the operational speed, for example the printing speed. When the arrest is released earlier, the lifting sucker may touch the trailing edge of the paper sheet which may damage the same or the paper sheets may be torn away from the following transport means or at least shifted in their position. This is particularly critical if the following transport devices are entraining suckers.
A proposed solution for simple, brief stopping of lifting suckers is known from German Patent DD PS 293 562 A5, corresponding to U.S. Pat. No. 5,064,184 to Liepert. The lifting sucker is thereby retained in the upper position by means of suction air via an additional suction line in the suction head which acts from above on a flat contact surface of the lifting sucker. After the rear edge of the paper sheet taken over by the entraining suckers leaves the stroke region of the lifting sucker, the suction air is interrupted in the additional suction line. The lifting sucker can be lowered for accepting the next paper sheet in a conventional manner. Due to the additional suction line disposed in the close vicinity of the lifting sucker and to the suction control, the proposed lifting sucker is very expensive and complicated. The retaining surface is shaped as a nose, so that a rotation of the sucker, as it is often required during operation, is not possible without limitations to the quickly reacting holding mechanism. Also, the pneumatic forces must be adjusted very accurately to the weight of the lifting sucker.